Drug, vitamin, diagnostic, and imaging agent conjugates have been used to treat, diagnose, and evaluate various disease states in humans and in animals.
In many cases these drug, vitamin, diagnostic, and imaging agent conjugates include intervening linkers separating for example a targeting ligand from a drug, diagnostic agent, or imaging agent. These linkers include a wide variety of bivalent fragments that may be used separately or when linked together for inclusion in conjugates to for example space a drug, diagnostic agent, or imaging apart from other parts of the conjugate, such as for example a vitamin or other targeting ligand. These linkers may also be stable to the metabolic, physiological, or biological conditions present when they are administered to humans and animals, or alternatively these linkers may undergo various routes of cleavage and/or fragmentation under such conditions. There exists a continuing need for bivalent linkers that can be generally used in drug, vitamin, diagnostic, and imaging agent conjugates.